by Jim Corbett, USA TODAY Sports

They also approved a ban on overloading the line of scrimmage with more than six players on one side of the center during PATs and field goals.

Both proposals passed by the required number of 24 of 32 votes. A flag drawn for violating either of the new rules will result in a 15-yard penalty.

Cushing's 2012 season was ended Oct. 8 by a shredded left anterior cruciate ligament suffered on an peel-back block by New York Jets guard Matt Slauson.

Now such blocks are illegal inside the tackle box as well, meaning a blocker can no longer cut down an opponent at the knees from the blindside anywhere on the field. Slauson cut Cushing down on a blindside hit when he was pursuing Jets running back Bilal Powell outside the tackle box.

"Really, it wasn't about trying to block the kick. It was about taking a shot at the offensive linemen," he said. "So now you can't go low. And you can't put four guys over one guy. And the center, when he has to snap the ball, that's just a vulnerable position for a helmet-to-helmet hit.

"These are two really good rules changes.''

Harbaugh said both rules proposals were recommended for safety reasons by the Players Association and the players sub-committee that spoke to the competition committee during the February scouting combine.

Cushing, who is expected to begin running this week, told USA TODAY Sports, "If my injury further prevents other injuries, then that's success, and there can be some good to come out of my injury. Hopefully, my injury does change the rule and in the future will prevent tons and tons of knee injuries.

"It's just asking for an injury when you get guys this big, this fast going full speed at legs. What do you think is going to happen?''

The most hotly-debated issue, preventing running backs from ducking their heads and using the crown of the helmet as a weapon is still under discussion along with two other rules proposals.